Microwave electromagnetic and absorption properties of some LiZn ferrites

The complex scattering parameters (S-11(*), S-21(*), S-12(*) and S-22(*)), microwave dielectric permittivity (epsilon(r)(*) = epsilon(r)' - jepsilon(r)"), magnetic permeability (mu(r)(*) = mu(r)(') - jmu(r)") and absorption characteristics of an undoped and doped samples (1 wt% CuO and MgO) of (Li0.5Fe0.5)(0.7)Zn0.3Fe2O4 (LiZn ferrite) were measured using a microwave vector network analyzer by means of coaxial two-port and short-S-11 techniques in the frequency range of 0.3-13.51 GHz. The addition of CuO and MgO has been found to increase the reflectivity and transmittivity relative to the pure LiZn ferrite. The dielectric constant and loss decrease with the oxide addition but the change in the magnetic permeability is minimal. The ferrites exhibit magnetic resonance due to domain wall movement in the low-frequency regime. The specular absorber method is used in determining the reflection loss (R-L) of the absorber at different thicknesses. This would provide a feasible means of determining the frequency and thickness configuration for minimum reflection from the surface of an absorber. Microwave absorption profiles for all samples calculated using the specular absorber method are in good agreement with those obtained experimentally from the short-S-11 technique. The ferrites show a low-frequency matching condition when the complex relative dielectric permittivity equals that of the complex relative magnetic permeability (epsilon(r)(*) = mu(r)(*)). The matching frequency is found to increase but with a smaller matching thickness for the doped samples. The second matching condition at higher frequency, which occur due to a geometrical cancellation of the incident and reflected waves at the surface of the absorbers, can only be seen for the LiZn/MgO sample. The second matching condition for the other two samples is assumed to occur at higher frequency. (C) 2003 Elsevier B.V. All rights reserved.